The present invention relates to a semiconductor laser device which is used for an optical disk and so on and to a manufacturing method thereof. More particularly, the present invention relates to a window structure semiconductor laser device which is especially excellent in characteristics of high output operation and to a manufacturing method thereof.
Recently, various kinds of semiconductor lasers have been widely used as a light source for an optical disk device. In Particular, a high output semiconductor laser has been used as a light source for write access to a disk such as a MD (Mini Disc) player and a CD-R (Compact Disc Recordable) drive, and is strenuously required to have higher output.
One of the factors which do not allow a semiconductor laser to have a higher output is catastrophic optical damage (COD). COD is generated when an optical output density increases in an active layer region in the vicinity of a laser resonator end surface which is served as a light emitting end surface.
Generation of the COD is attributed to the fact that the active region in the vicinity of the laser resonator end surface is a region for absorbing laser beams. In the laser resonator end surface, there are a number of non-radiative recombination centers of a surface level or an interface level. Carriers injected into the active layer in the vicinity of the laser resonator end surface are dissipated due to the non-radiative recombination, which makes density of the carriers injected in the active layer in the vicinity of the laser resonator end surface lower than that in a center portion thereof. As a result, the active layer region in the vicinity of the laser resonator end surface acts as an absorbing region for a wavelength of the laser beam formed with high injected carrier density in the central portion.
With higher optical output density, heat generation becomes larger locally in the absorbing region, which increases a temperature and thereby reduces bandgap energy. As a result, there is established positive feedback in which an absorption coefficient further increments and the temperature increases, which causes the temperature of the absorbing region in the vicinity of the laser resonator end surface to reach a melting point of the absorbing region, thereby generating COD.
As one of methods for allowing a semiconductor laser to have a higher output in order to improve the COD level, there has been employed a method for utilizing a window structure through disordering a multiquantum well structure active layer as disclosed in Japanese Patent Laid-Open Publication HEI No. 9-23037.
For showing a prior art example of semiconductor lasers having the window structure, structural drawings of a semiconductor laser device described in Japanese Patent Laid-Open Publication No. H9-23037 are provided in FIGS. 12A to 12C.
FIG. 12A is a perspective view including a laser resonator end surface. FIG. 12B is a cross sectional view taken along a line Ia-Iaxe2x80x2 of FIG. 12A showing a wave guide. FIG. 12C is a cross sectional view in layer thickness direction taken along a line Ib-Ibxe2x80x2 of FIG. 12A. In FIGS. 12A to 12C, there are shown a GaAs substrate 1001, an n-type AlGaAs lower cladding layer 1002, a quantum well active layer 1003, a p-type AlGaAs first upper cladding layer 1004a, a p-type AlGas second upper cladding layer 1004b, a p-type Gas contact layer 1005, a vacancy diffusion region 1006 (shaded portion), a proton injection region 1007 (shaded portion), a negative electrode 1008, a positive electrode 1009, a laser resonator end surface 1020, a region 1003a of the quantum well active layer 1003 contributing to laser emission, and a window structure region 1003b of the quantum well active layer 1003 formed in the vicinity of the laser resonator end surface 1020.
Next, description will be given of a method for manufacturing conventional semiconductor laser devices with reference to a flowchart shown in FIG. 13.
On an n-type GaAs substrate 1001, there are epitaxially grown an n type AlGaAs lower cladding layer 1002, a quantum well active layer 1003, and a p-type AlGaAs first upper cladding layer 1004a in sequence (FIG. 13A). Next, an SiO2 film 1010 is formed on the surface of the p-type AlGaAs first upper cladding layer 1004a. Further, a stripe-shaped opening 1010a is formed in the SiO2 film 1010 in such a way to extend in laser resonator direction but not to reach a laser resonator end surface (FIG. 133) A thus-prepared wafer is annealed at a temperature of 800xc2x0 C. or more in an As atmosphere. During annealing, the SiO2 film 1010 absorbs Ga atoms from the surface of the adjacent p-type AlGaAs first upper cladding layer 1004a, so that Ga vacancies are formed within the p-type AlGaAs first upper cladding layer 1004a. Some of the Ga vacancies diffuse till they reach the quantum well active layer 1003 inside the crystal, thereby causing disorder of the quantum well structure. In a disordered active layer region, an effective forbidden bandwidth extends, and therefore the disordered active layer region functions as a transparent window for emitted laser beans.
Then, the SiO2 film 1010 is removed, and there are epitaxially grown a p-type AlGaAs second upper cladding layer 1004b and a p-type GaAs contact layer 1005 in sequence on the p-type AlGaAs first upper cladding layer 1004a (FIG. 13C). Next, a resist film is formed on the p-type GaAs contact layer 1005a. Then, a stripe-shaped resist 1011 is formed by photolithographic technique above the same region as the stripe-shaped opening 1010a of the SiO2 film 1010. Then, with the stripe-shaped resist 1011 as a mask, protons are injected thorough the surface of the p-type GaAs contact layer 1005 into the p-type AlGaAs second upper cladding layer 1004b so as to form a high resisting region 1007 which serves as a current blocking layer (FIG. 13D). Finally, a negative electrode 1008 is formed on the GaAs substrate 1001, and a positive electrode 1009 is formed on the p-type GaAs contact layer 1005. The thus-completed wafer is cleaved to provide the semiconductor laser device of FIG. 12.
In the case of the conventional window structure semiconductor laser device, an SiO2 film 1010 is formed on the surface of the p-type AlGaAs first upper cladding layer 1004a so that bandgap energy In the disordered region formed in the vicinity of the laser resonator end surface may become larger than bandgap energy corresponding to a laser emission wavelength. By formation of the SiO, film 1010, Ga vacancies are created in the p-type AlGas first upper cladding layer 1004a adjacent to the Sio2 film 1010, and some of the Ga vacancies are diffused into the quantum well active layer 1003.
Creation and diffusion of the Ga vacancies are conducted in the region covered with the SiO2 film 1010. However, annealing at a temperature of 800xc2x0 C. or more causes creation of Ga vacancies, though small in amount due to re-evaporation of Ga atoms, in the uppermost surface of the region not covered with he SiO2 film 1010 (region inside the laser resonator). As a result, the Ga vacancies are diffused into the quantum well active layer 1003.
Consequently, the bandgap of the quantum well active layer 1003 fluctuates inside the laser resonator, and the long-term reliability of the quantum well active layer 1003 is degraded due to deterioration of crystallinity.
If annealing temperature is lower or if annealing time is shorter as an countermeasure against the above problem, diffusion of Ga vacancies to the quantum well active layer 1003 may be inhibited inside the laser resonator. However, it becomes insufficient to create vacancies in the region covered with the SiO2 film 1010 and to diffuse the vacancies to the quantum well active layer 1003 in the region covered with the SiO2 film 1010. This results in adsorption of laser beams in the vicinity of the laser resonator end surface. As a result, COD is easily generated in the active layer region in the vicinity of the laser resonator end surface, which causes reduced maximum optical output in high output driving, and prevents implementation of sufficient long-term reliability.
It is an object of the present invention to provide a semiconductor laser device which inhibits fluctuation of the bandgap of an active layer inside a laser resonator and has excellent long-term reliability, as well as a method for manufacturing the same.
In order to accomplish the above object, the present invention provides a semiconductor laser device having a quantum well active layer including a well layer and a barrier layer laminated on a semiconductor substrate, in which a photoluminescence light from the quantum well active layer in a vicinity of a light emitting end surface is smaller in a peak wavelength than a photoluminescence light from the quantum well active layer in an internal region, wherein
each of the well layer and the barrier layer of the quantum well active layer contains II group atoms.
According to the above constitution, the semiconductor laser device is so formed that the peak wavelength of the photoluminescence light from the quantum well active layer in the vicinity of the light emitting end surface is smaller than the peak wavelength of the photoluminescence light from the quantum well active layer in the internal region. This indicates that a bandgap of the quantum well active layer in the vicinity of the light emitting end surface is larger than a bandgap of the quantum well active layer in the internal region thereof.
The larger bandgap of the quantum well active layer in the vicinity of the light emitting end surface is achieved by vacancies which are introduced into the quantum well active layer in the vicinity of the light emitting end surface for example by annealing. Vacancies, though small in amount, are simultaneously introduced into the internal region of the quantum well active layer by annealing. These vacancies may cause fluctuation of the bandgap of the quantum well active layer in the internal region thereof and deterioration of crystallinity.
However, in the present invention, each of the well layer and the barrier layer of the quantum well active layer contains the II group atoms. Therefore, the II group atoms residing inside the quantum well active layer complement i.e. make up for vacancies in the internal region of the quantum, well active layer. Disappearance of vacancies in the internal region of the quantum well active layer prevents fluctuation of the bandgap of the quantum well active layer in the internal region thereof.
In one embodiment of the present invention, the semiconductor laser device further comprises two cladding layers for interposing the quantum well active layer therebetween, wherein
impurity atoms contained in the cladding layers are identical to the II group atoms.
The II group atoms in the well layer and the barrier layer of the quantum well active layer may be supplied by diffusion of the impurity atoms from the cladding layers that interpose the quantum well active layer therebetween for example by annealing.
In one embodiment of The present invention, an impurity atom density of the II group atoms contained in the well layer is within a range from 3xc3x971017 cmxe2x88x923 to 1xc3x971013 cmxe2x88x923.
According to the present embodiment, the density of II group atoms in the well layer ranged from 3xc3x971017 cmxe2x88x923 to 1xc3x971018 cmxe2x88x923 makes it possible to prevent shift of the II group atoms to a cladding layer on the side of the substrate, and therefore to prevent shift of a p-n joint position to the side of the substrate. As a result, overflow of carriers from the quantum well active layer may be inhibited, and a semiconductor laser device with decreased driving current in high output may be implemented.
In one embodiment of the present invention, Si atoms are contained in a first cladding layer for interposing the quantum well active layer from a side of the semiconductor substrate, of the two cladding layers for interposing the quantum well active layer therebetween.
According to the present embodiment, the Si atoms contained in the first cladding layer make it possible to inhibit diffusion of the II group atoms because similar to the II group atoms, Si atoms tend to reside in III group atom sites. Consequently, driving current in high output may be decreased, and a semiconductor Laser device having excellent long-term reliability in high output driving may be implemented.
In one embodiment of the present invention, the II group atoms are contained in a second cladding layer for interposing the quantum well active layer from a side opposite to the side of the semiconductor substrate.
In one embodiment of the present invention, the semiconductor laser device further comprises ridge-shaped stripe geometry extended in a resonance direction and formed on a second cladding layer for interposing the quantum well active layer from a side opposite to a side of the semiconductor substrate, and wherein
a current non-injection area selectively formed in a region in a vicinity of a light emitting end surface on the ridge-shaped stripe geometry.
According to the present embodiment, the current non-injection area is selectively formed in the region in the vicinity of the light emitting end surface on the ridge-shaped stripe geometry formed in the second cladding layer. The current non-injection area prevents electric current from flowing into a window region of the quantum well active layer in the region in the vicinity of the light emitting end surface, and inhibits carrier loss due to the presence of vacancy defects in the window region. Thereby, ineffective current not contributing to light emission is reduced. Consequently, driving current in high output may be decreased, and a semiconductor laser device may have excellent long-term reliability in high output driving.
In one embodiment of the present invention, the semiconductor substrate is composed of GaAs, and
a semiconductor layer composed of at least an AlGaAs based material is laminated on the semiconductor substrate.
In one embodiment of the present invention, the semiconductor substrate is composed of GaAs, and
a semiconductor layer composed of at least an AlGaInP based material is laminated on the semiconductor substrate.
In one embodiment of the present invention, the II group atom is any one of a zinc atom, a beryllium atom and a magnesium atom.
According to the present embodiment, any one of a zinc atom, a beryllium atom and a magnesium atom is used as the II group atom. Those II group atoms may effectively complement the vacancies diffused into the quantum well active layer.
The present invent-on also provides a method for manufacturing a semiconductor laser device, comprising the steps of:
growing a laminated structure having a first conductivity type cladding layer, a quantum well active layer composed of a well layer and a barrier layer including II group atoms, and a second conductivity type cladding layer on a first conductivity type semiconductor substrate;
selectively forming a dielectric film in a vicinity of a light emitting end surface on the laminated structure; and
making a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region in which the dielectric film is formed smaller than a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region in which the dielectric film is not formed, by annealing.
According to the above constitution, II group atoms are added in advance when the quantum well active layer grows inside the laminated structure formed on a first conductivity type semiconductor substrate. After the II group atoms are contained in the quantum well active layer, annealing is conducted with a dielectric film applied only to the region in the vicinity of the light emitting end surface of the laminated structure. As a result, constitutive atoms are absorbed from the surface of the laminated structure right under the dielectric film into the dielectric film, so that vacancies are created inside the laminated structure to diffuse into the quantum well active layer in the vicinity of the light emitting end surface. As a result, the bandgap of the quantum well active layer in the region in the vicinity of the light emitting end surface becomes larger than the bandgap in the internal region thereof.
In annealing, a small amount of vacancies are created also in the internal region on the surface of the laminated structure and are diffused into in the internal region of the quantum well active layer. However, the II group atoms contained in the quantum well active layer complement the small amount of vacancies, and therefore fluctuation of the bandgap of the quantum well active layer in the internal region thereof is inhibited. Further, because the quantum well active layer contains II group atoms prior to annealing, density inclination of the II group atoms in the vicinity of the quantum well active layer is small, which inhibits diffusion of II group atoms into the quantum well active layer caused by the annealing. Therefore, density increase of the II group atoms in the quantum well active layer in the internal region is inhibited, thereby making it possible to prevent deterioration of crystallinity of the quantum well active layer in the internal region.
Furthermore, because there is no density increase of the II group atoms in the quantum well active layer caused by the annealing as described above, diffusion of the II group atoms from the quantum well active layer into the first conductivity type cladding layer due to the annealing is inhibited, thereby making it possible to prevent overflow of carriers from the quantum well active layer in high output driving.
The present invention further provides a method for manufacturing a semiconductor laser device, comprising the steps of:
growing a laminated structure having a first conductivity type cladding layer, a quantum well active layer composed of a well layer and a barrier layer, and a second conductivity type cladding layer including II group atoms on a first conductivity type semiconductor substrate;
diffusing the II group atone in the second conductivity type cladding layer into the quantum well active layer by first annealing;
selectively forming a dielectric film in a region in a vicinity of a light emitting end surface on the laminated structure; and
making a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region in which the dielectric film is formed smaller than a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region in which the dielectric film Is not formed, by second annealing.
According to the above constitution, when the II group atoms are added to the quantum well active layer, the II group atoms in the second conductivity type cladding layer are diffused into the quantum well active layer by annealing. There are, therefore, conducted two annealing steps including one which is conducted after a dielectric film is selectively applied. This equalizes a distribution of the II group atoms in the region in the vicinity of the quantum well active layer, and inhibits diffusion of the II group atoms into the first conductivity type cladding layer due to the annealing, thereby making it possible to prevent overflow of carriers from the quantum well active layer in high output driving.
In addition, constitutive atoms are absorbed from the surface of the laminated structure right under the dielectric film formed in the region in the vicinity of the light emitting end surface into the dielectric film, and vacancies are created inside the laminated structure, which promotes diffusion of the vacancies into the quantum well active layer. Here, because II group atoms are contained in the quantum well active layer, the II group atoms complement a small amount of vacancies, which are created in annealing on the surface of the laminated structure in the internal region and are diffused into the quantum well active layer. Therefore, fluctuation of the bandgap of the quantum well active layer in the internal region is inhibited.
The present invention also provides a method for manufacturing a semiconductor laser device, comprising the steps of:
growing a laminated structure having a first conductivity type cladding layer, a quantum well active layer composed of a well layer and a barrier layer including II group atoms, and a second conductivity type cladding layer on a first conductivity type semiconductor substrate;
selectively irradiating a region in a vicinity of a light emitting end surface of the laminated structure with ionized atoms; and
making a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region which is irradiated with the ionized atoms smaller than a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region which is not irradiated with ionized atoms, by annealing.
According to the above constitution, II group atoms are added in advance when the quantum well active layer grows inside the laminated structure formed on a first conductivity type semiconductor substrate. After the II group atoms are contained in the quantum well active layer, only the region in the vicinity of the light emitting end surface on the laminated structure are irradiated with ionized atoms. As a result, vacancies are created on the surface of the laminated structure in the region irradiated with the ionized atoms, and diffusion of the vacancies into the quantum well active layer is promoted by annealing. As a result, the bandgap of the quantum well active layer in the region in the vicinity of the light emitting end surface becomes larger than the bandgap in the internal region.
Because II group atoms are contained in the quantum well active layer, the II group atoms complement a small amount of vacancies, which are created in annealing on the surface of the laminated structure in the internal region and are diffused into the quantum well active layer. Therefore, fluctuation of the bandgap of the quantum well active layer in the internal region is inhibited. Further, because the quantum well active layer contains II group atoms prior to annealing, density inclination of II group atoms in the vicinity of the quantum well active layer is small, which inhibits diffusion of II group atoms into the quantum well active layer caused by the annealing. Therefore, density increase of the II group atoms in the quantum well active layer in the internal region is inhibited, thereby making it possible to prevent deterioration of crystallinity of the quantum well active layer in the internal region.
Furthermore, because there is no density increase of the II group atoms in the quantum well active layer caused by the annealing as described above, diffusion of the II group atoms from he quantum well active layer into the first conductivity type cladding layer due to the annealing is inhibited, thereby making it possible to prevent overflow of carriers from the quantum well active layer in high output driving.
The present invention further provides a method for manufacturing a semiconductor laser device, comprising the steps of:
growing a laminated structure having a first conductivity type cladding layer, a quantum well active layer composed of a well layer and a barrier layer, and a second conductivity type cladding layer including II group atoms on a first conductivity type semiconductor substrate;
diffusing the II group atoms in the second conductivity type cladding layer into the quantum well active layer by first annealing;
selectively irradiating a region in a vicinity of a light emitting end surface of the laminated structure with ionized atoms; and
making a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region which is irradiated with the ionized atoms smaller than a peak wavelength of a photoluminescence light from the quantum well active layer beneath a region which is not irradiated with ionized atoms, by second annealing.
According to the above constitution, when the II group atoms are added to the quantum well active layer, the II group atoms in the second conductivity type cladding layer are diffused into the quantum well active layer by annealing. There are, therefore, conducted two annealing steps including one which is conducted after selective irradiation with the ionized atoms. This equalizes a distribution of the II group atoms in the region in the vicinity of the quantum well active layer, and inhibits diffusion of the II group atoms into the first conductivity type cladding layer caused by the annealing, thereby making it possible to prevent overflow of carriers from the quantum well active layer in high output driving.
Furthermore, ion irradiation of the region in the vicinity of the light emitting end surface on the laminated structure creates vacancies on the surface of the laminated structure, and diffusion of the vacancies into he quantum well active layer is promoted by annealing. Here, because II group atoms are contained in the quantum well active layer, the II group atoms complement a small amount of vacancies, which are created in annealing on the surface of the laminated structure in the internal region and are diffused into the quantum well active layer. Therefore, fluctuation of the bandgap of she quantum well active layer in the internal region is inhibited.
In one embodiment of the present invention, a dielectric film is used as a mask when the region in the vicinity of the light emitting end surface of the laminated structure is selectively irradiated with the ionized atoms.
According to the above constitution, using a dielectric film as a mask in irradiation with the ionized atoms implements low driving voltage compared with the case of using a resist.
In one embodiment of the present invention, the ionized atom is at least any one of argon, oxygen and nitrogen.
According to the above constitution, vacancies are effectively created on the surface of the laminated structure in the region in the vicinity of the light emitting end surface, and diffusion thereof into the quantum well active layer due to the annealing is promoted. As a result, disorder of the region in the vicinity of the light emitting end surface in the quantum well active layer is accelerated.
In one embodiment of the present invention, the II group atom is any one of a zinc atom, a beryllium atom and a magnesium atom.
According to the present embodiment, as the II group atom, any one of a zinc atom, a beryllium atom, and a magnesium atom is used and created in the cladding layer in the internal region, thereby enabling effective complementation between the vacancies to be diffused into the quantum well active layer and the II group atoms.
The semiconductor laser devices suitable for the present invention are required to have a quantum well active layer. Between the quantum well active layer and the cladding layer, there is preferably disposed a light guiding layer. The reason hereof is described below. In the case of disposing the light guiding layer, the light guiding layer is regarded as a part of the quantum well active layer of the present invention.
In a multiquantum well active layer without a light guiding layer, a radiation angle in vertical direction is too large, which causes problems in an optical characteristic, and hinders application to LD for disks.
The multiquantum well active layer without a light guiding layer also suffers abnormally high power density in the vicinity of the laser resonator end surface, i.e., the region in the vicinity of the light emitting end surface, as well as in the internal region, in addition to intense adsorption of laser beams, which tends to increase crystal defect. It is impossible to solve these problems even if the window structure is employed.
However, composing the quantum well active layer by interposing a multiquantum well between the light guiding layers makes it possible to alleviate the severity of the above-stated problems.
The function of the light guiding layer of the present invention is to make the II group atoms to complement vacancies, which are to be diffused into the quantum well layer inside the laser resonator, in a light guiding layer containing II group atoms, so as to reduce the amount of the vacancies diffused into the quantum well layer as much as possible. This makes it possible to ensure that the fluctuation of the bandgap of the active layer inside the laser resonator is inhibited.
In order to accomplish the present invention, the semiconductor laser device is preferably structured with the following constitutions.
1. Magnitude correlation of the bandgap is:
quantum well active layer less than N-type cladding layerxe2x89xa6P-type cladding layer
2. The number of quantum well layers composing an active layer is 2 to 3 layers.
3. The thickness of a quantum well layer is 50 to 100 xc3x85.
4. The light guiding layer adjacent to the N-type cladding layer may not contain II group atoms.
The method for manufacturing the above-stated semiconductor laser device having different bandgaps provided by irradiation with fine ionized atom is preferably further composed of the steps of:
forming another laminated structure including a second conductivity type etching stop layer, another second conductivity type cladding layer, and a second conductivity type protective layer, on the existing laminated structure on the semiconductor substrate;
forming a dielectric film on the laminated structure as an ion irradiation mask;
after making the bandgap in the vicinity of the resonator end surface of the quantum well active layer included in the laminated structure larger than the bandgap in the region inside the resonator by irradiation with ionized atoms,
forming a ridge-shape strip extending in resonator direction on a layer formed above the etching stop layer;
developing a first conductivity type current blocking layer on the semiconductor substrate including the ridge-shaped stripe; and
removing the first conductivity type current blocking layer on the dielectric film processed to be the ridge-shaped stripe.
Because the first conductivity type current blocking layer formed on the dielectric film on the laminated structure grows to have physical characteristics different from that of the first conductivity type current blocking layer formed on a layer other than the dielectric film, it is easily removed by etching.
Thus, in the ridge-shaped stripe in laser resonator direction, there may be created a current non-injection area only in the vicinity of the end surface thereof with simple constitutions.